The present invention relates to a high power electron tube and particularly to an improved grid structure for such a tube.
A high-power electron tube such as the RCA 8916 VHF linear power amplifier tube may be used in high gain, high linearity equipments for VHF-TV and FM service and for communication transmitters to 400 MHz. The terminals of such a power tube are coaxial for operation in a normal coaxial TEM mode. In the operation of such a power tube there are an infinite number of spurious resonant modes, such as the TE and TM modes, that may exist simultaneously within a resonant cavity or system along with the desired TEM mode. In a typical power tube the spurious resonances occur at frequencies that are generally much higher than the desired operating frequency of the tube and thus cause no problems because little or no power can be generated in the tube at these frequencies. However, there are generally a few spurious modes that do pose problems and these problems are manifested as rf oscillations at the spurious mode frequencies. These rf oscillations are detrimental to tube performance in that they can increase any of the following tube operating parameters: peak operating voltage; unit area electrode dissipation; dielectric losses; noise on signal; and rf leakage; as well as generate interference signals, and reduce circuit operating efficiency. In some instances tube failures have been reported due to cracks in ceramic insulators attributable to the aforementioned rf oscillations.
For the detrimental effects of the spurious rf oscillations to occur there must be rf feedback from an output circuit comprising the anode and associated electrodes to an input circuit comprising the control grid and associated electrodes of such a magnitude and phase to overcome any inherent circuit losses. If these conditions are fulfilled, the strength of the spurious oscillations will increase until the spurious losses equal the power generated at the spurious frequencies. It has been determined that rf feedback is dictated by such factors as grid aperture length, grid thickness and aperture orientation.
Attempts to eliminate the causes of these undesirable rf oscillations have been largely unsuccessful and instead efforts are generally made to attenuate spurious oscillations, at least to a level wherein the amplitudes are small compared to the desired operating frequency. For example, contemporary power tube circuits generally oscillate at frequencies that are associated with the TE or TM circular modes. Typically these modes inherently have low loss characteristics and attempts to provide significant circular mode loading are often frustrated by the inability to achieve circular mode loading without reducing circuit efficiency at the desired operating frequency.
Since it is difficult to specifically load the spurious circular modes without degrading the designed amplifier characteristics of the tube, it becomes attractive to prevent or control the onset of spurious oscillations by reactively attentuating the spurious rf feedback.